1. Field of the Invention
This invention relates to a glow plug, and more particularly toward a glow plug having an integrated pressure sensing device for use in an internal combustion engine.
2. Related Art
Glow plugs are typically used in applications where a source of intense heat is required to either directly initiate or to aid in the initiation of combustion. As such, glow plugs are used in space heaters, industrial furnaces and diesel engines to name a few.
In the field of compression ignition engines, there are trends toward ever greater output and efficiency, as well as toward the use of flexible fuels, which together have increased the demand for and usage of various types of combustion sensors necessary to enable enhanced control of the engine and combustion processes. Combustion sensors, particularly combustion pressure sensors, have in the past been discrete sensors that are inserted into the combustion chamber through special threaded bores created just to accommodate these sensors. The sensors themselves have generally been used only in engine and engine control development, and not in mass production owing to their high cost and the additional demands they place on space around the cylinder head.
Several examples of glow plugs with integrated pressure sensors can be found in the prior art. A particular problem or concern with many, if not all, pressure sensing glow plug designs relates to the undesirable stresses introduced into the glow plug components, and particularly to the electrode itself, in order to adequately preload the pressure sensor. Joint strength between the electrode and heater probe components is quite often challenged by prior art designs which incorporate a pressure sensing device into the glow plug shell.
One prior art example may be found in US Publication No. 2007/0095811 published May 3, 2007. According to this design, a pressure sensing unit is preinstalled on the heater probe and pretensioned through an external support tube that is subsequently joined to the glow plug shell at its upper end. A particular drawback of this arrangement lies in the way its flexible membrane element between the glow plug shell and heater probe (to accommodate pressure fluctuations) is compressed along its length. A further drawback of this design resides in the location of its sensor element which protrudes into the combustion gas relatively far away from the cylinder head seat, and is thus subject to rapid thermal shock. These features lead to reduced working life and less than optimal functionality.
Taken as a whole, prior art glow plugs with integrated pressure sensors tend to place the center electrode or other force transmitting member in tension, with the shell components in compression. FIG. 2 provides an illustration of one such prior art glow plug design. The joint between the center electrode and the heater probe needs a tensile strength which is not required in normal glow plug operations, and which is very difficult to achieve. Furthermore, preloads or pretensioning on the sensor must be high enough to ensure that load always stays on the sensor under all conditions, even as changes in the sensed pressure reduce the preload. Doubtless, some random examples do exist where the center electrode is not tensioned, such as in the above-noted US 2007/0095811. However, these examples are prone to distortions and other design defects. Prior art designs also have a certain minimum length required for all necessary components, and rely on forces transmitted through the long and thin center electrode which can give problems of thermal performance and reduced sensitivity. Furthermore, manufacturing issues related to the assembly of a sensor stack, i.e., the stack of components which together function as a sensor assembly, complicate the necessary electrical connections.
Accordingly, there is a need for a glow plug with integrated pressure sensor that avoids placing unnecessary stress on the center electrode component, enables lower starting loads, better thermal performance, higher sensitivity, and does not require a strong bond from center electrode to heater probe. Furthermore, there is a need for such a glow plug and pressure sensor assembly that is more easily assembled in the context of high volume production.